(libc.info.gz) Low-Level Time String Parsing
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(libc.info.gz) Parsing Date and Time
(libc.info.gz) General Time String Parsing
21.4.6.1 Interpret string according to given format
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The first function is rather low-level. It is nevertheless frequently
used in software since it is better known. Its interface and
implementation are heavily influenced by the `getdate' function, which
is defined and implemented in terms of calls to `strptime'.
-- Function: char * strptime (const char *S, const char *FMT, struct
tm *TP)
The `strptime' function parses the input string S according to the
format string FMT and stores its results in the structure TP.
The input string could be generated by a `strftime' call or
obtained any other way. It does not need to be in a
human-recognizable format; e.g. a date passed as `"02:1999:9"' is
acceptable, even though it is ambiguous without context. As long
as the format string FMT matches the input string the function
will succeed.
The user has to make sure, though, that the input can be parsed in
a unambiguous way. The string `"1999112"' can be parsed using the
format `"%Y%m%d"' as 1999-1-12, 1999-11-2, or even 19991-1-2. It
is necessary to add appropriate separators to reliably get results.
The format string consists of the same components as the format
string of the `strftime' function. The only difference is that
the flags `_', `-', `0', and `^' are not allowed. Several of the
distinct formats of `strftime' do the same work in `strptime'
since differences like case of the input do not matter. For
reasons of symmetry all formats are supported, though.
The modifiers `E' and `O' are also allowed everywhere the
`strftime' function allows them.
The formats are:
`%a'
`%A'
The weekday name according to the current locale, in
abbreviated form or the full name.
`%b'
`%B'
`%h'
The month name according to the current locale, in
abbreviated form or the full name.
`%c'
The date and time representation for the current locale.
`%Ec'
Like `%c' but the locale's alternative date and time format
is used.
`%C'
The century of the year.
It makes sense to use this format only if the format string
also contains the `%y' format.
`%EC'
The locale's representation of the period.
Unlike `%C' it sometimes makes sense to use this format since
some cultures represent years relative to the beginning of
eras instead of using the Gregorian years.
`%d'
`%e'
The day of the month as a decimal number (range `1' through
`31'). Leading zeroes are permitted but not required.
`%Od'
`%Oe'
Same as `%d' but using the locale's alternative numeric
symbols.
Leading zeroes are permitted but not required.
`%D'
Equivalent to `%m/%d/%y'.
`%F'
Equivalent to `%Y-%m-%d', which is the ISO 8601 date format.
This is a GNU extension following an ISO C99 extension to
`strftime'.
`%g'
The year corresponding to the ISO week number, but without
the century (range `00' through `99').
_Note:_ Currently, this is not fully implemented. The format
is recognized, input is consumed but no field in TM is set.
This format is a GNU extension following a GNU extension of
`strftime'.
`%G'
The year corresponding to the ISO week number.
_Note:_ Currently, this is not fully implemented. The format
is recognized, input is consumed but no field in TM is set.
This format is a GNU extension following a GNU extension of
`strftime'.
`%H'
`%k'
The hour as a decimal number, using a 24-hour clock (range
`00' through `23').
`%k' is a GNU extension following a GNU extension of
`strftime'.
`%OH'
Same as `%H' but using the locale's alternative numeric
symbols.
`%I'
`%l'
The hour as a decimal number, using a 12-hour clock (range
`01' through `12').
`%l' is a GNU extension following a GNU extension of
`strftime'.
`%OI'
Same as `%I' but using the locale's alternative numeric
symbols.
`%j'
The day of the year as a decimal number (range `1' through
`366').
Leading zeroes are permitted but not required.
`%m'
The month as a decimal number (range `1' through `12').
Leading zeroes are permitted but not required.
`%Om'
Same as `%m' but using the locale's alternative numeric
symbols.
`%M'
The minute as a decimal number (range `0' through `59').
Leading zeroes are permitted but not required.
`%OM'
Same as `%M' but using the locale's alternative numeric
symbols.
`%n'
`%t'
Matches any white space.
`%p'
`%P'
The locale-dependent equivalent to `AM' or `PM'.
This format is not useful unless `%I' or `%l' is also used.
Another complication is that the locale might not define
these values at all and therefore the conversion fails.
`%P' is a GNU extension following a GNU extension to
`strftime'.
`%r'
The complete time using the AM/PM format of the current
locale.
A complication is that the locale might not define this
format at all and therefore the conversion fails.
`%R'
The hour and minute in decimal numbers using the format
`%H:%M'.
`%R' is a GNU extension following a GNU extension to
`strftime'.
`%s'
The number of seconds since the epoch, i.e., since 1970-01-01
00:00:00 UTC. Leap seconds are not counted unless leap
second support is available.
`%s' is a GNU extension following a GNU extension to
`strftime'.
`%S'
The seconds as a decimal number (range `0' through `60').
Leading zeroes are permitted but not required.
*NB:* The Unix specification says the upper bound on this
value is `61', a result of a decision to allow double leap
seconds. You will not see the value `61' because no minute
has more than one leap second, but the myth persists.
`%OS'
Same as `%S' but using the locale's alternative numeric
symbols.
`%T'
Equivalent to the use of `%H:%M:%S' in this place.
`%u'
The day of the week as a decimal number (range `1' through
`7'), Monday being `1'.
Leading zeroes are permitted but not required.
_Note:_ Currently, this is not fully implemented. The format
is recognized, input is consumed but no field in TM is set.
`%U'
The week number of the current year as a decimal number
(range `0' through `53').
Leading zeroes are permitted but not required.
`%OU'
Same as `%U' but using the locale's alternative numeric
symbols.
`%V'
The ISO 8601:1988 week number as a decimal number (range `1'
through `53').
Leading zeroes are permitted but not required.
_Note:_ Currently, this is not fully implemented. The format
is recognized, input is consumed but no field in TM is set.
`%w'
The day of the week as a decimal number (range `0' through
`6'), Sunday being `0'.
Leading zeroes are permitted but not required.
_Note:_ Currently, this is not fully implemented. The format
is recognized, input is consumed but no field in TM is set.
`%Ow'
Same as `%w' but using the locale's alternative numeric
symbols.
`%W'
The week number of the current year as a decimal number
(range `0' through `53').
Leading zeroes are permitted but not required.
_Note:_ Currently, this is not fully implemented. The format
is recognized, input is consumed but no field in TM is set.
`%OW'
Same as `%W' but using the locale's alternative numeric
symbols.
`%x'
The date using the locale's date format.
`%Ex'
Like `%x' but the locale's alternative data representation is
used.
`%X'
The time using the locale's time format.
`%EX'
Like `%X' but the locale's alternative time representation is
used.
`%y'
The year without a century as a decimal number (range `0'
through `99').
Leading zeroes are permitted but not required.
Note that it is questionable to use this format without the
`%C' format. The `strptime' function does regard input
values in the range 68 to 99 as the years 1969 to 1999 and
the values 0 to 68 as the years 2000 to 2068. But maybe this
heuristic fails for some input data.
Therefore it is best to avoid `%y' completely and use `%Y'
instead.
`%Ey'
The offset from `%EC' in the locale's alternative
representation.
`%Oy'
The offset of the year (from `%C') using the locale's
alternative numeric symbols.
`%Y'
The year as a decimal number, using the Gregorian calendar.
`%EY'
The full alternative year representation.
`%z'
The offset from GMT in ISO 8601/RFC822 format.
`%Z'
The timezone name.
_Note:_ Currently, this is not fully implemented. The format
is recognized, input is consumed but no field in TM is set.
`%%'
A literal `%' character.
All other characters in the format string must have a matching
character in the input string. Exceptions are white spaces in the
input string which can match zero or more whitespace characters in
the format string.
*Portability Note:* The XPG standard advises applications to use
at least one whitespace character (as specified by `isspace') or
other non-alphanumeric characters between any two conversion
specifications. The GNU C Library does not have this limitation
but other libraries might have trouble parsing formats like
`"%d%m%Y%H%M%S"'.
The `strptime' function processes the input string from right to
left. Each of the three possible input elements (white space,
literal, or format) are handled one after the other. If the input
cannot be matched to the format string the function stops. The
remainder of the format and input strings are not processed.
The function returns a pointer to the first character it was
unable to process. If the input string contains more characters
than required by the format string the return value points right
after the last consumed input character. If the whole input
string is consumed the return value points to the `NULL' byte at
the end of the string. If an error occurs, i.e., `strptime' fails
to match all of the format string, the function returns `NULL'.
The specification of the function in the XPG standard is rather
vague, leaving out a few important pieces of information. Most
importantly, it does not specify what happens to those elements of TM
which are not directly initialized by the different formats. The
implementations on different Unix systems vary here.
The GNU libc implementation does not touch those fields which are not
directly initialized. Exceptions are the `tm_wday' and `tm_yday'
elements, which are recomputed if any of the year, month, or date
elements changed. This has two implications:
* Before calling the `strptime' function for a new input string, you
should prepare the TM structure you pass. Normally this will mean
initializing all values are to zero. Alternatively, you can set
all fields to values like `INT_MAX', allowing you to determine
which elements were set by the function call. Zero does not work
here since it is a valid value for many of the fields.
Careful initialization is necessary if you want to find out
whether a certain field in TM was initialized by the function call.
* You can construct a `struct tm' value with several consecutive
`strptime' calls. A useful application of this is e.g. the parsing
of two separate strings, one containing date information and the
other time information. By parsing one after the other without
clearing the structure in-between, you can construct a complete
broken-down time.
The following example shows a function which parses a string which is
contains the date information in either US style or ISO 8601 form:
const char *
parse_date (const char *input, struct tm *tm)
{
const char *cp;
/* First clear the result structure. */
memset (tm, '\0', sizeof (*tm));
/* Try the ISO format first. */
cp = strptime (input, "%F", tm);
if (cp == NULL)
{
/* Does not match. Try the US form. */
cp = strptime (input, "%D", tm);
}
return cp;
}
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